FIG. 1 shows a conventional gas discharge laser design. The active medium in the FIG. 1 gas discharge laser 100 occupies the elongated area between two resonator mirrors; a high reflector mirror 102 and an output coupler mirror 104. The exciting of the active medium is performed by a gas discharge. The electrical circuit of the gas discharge includes two metal electrodes 106, 108 contacting with the laser gas medium and are placed parallel to each other. The length of the electrodes 106, 108 defines the length of the laser medium, while the separation, width and surface shape of the electrodes 106, 108 provide the proper cross section and properties of the laser beam.
Regular convex is the most popular electrode design. As mentioned above, the active surface shape and separation of the electrodes provide the required gas discharge dimensions. FIG. 2 shows the typical convex electrode arrangement; the well-known laser chamber containing gas in which the discharge is struck is not shown in FIG. 2 for convenience of illustration. As is well known, laser light travels along the longitudinal axis of the resonator.
FIG. 3 shows step electrodes, that is, a modification of the FIG. 2 regular convex electrodes with a relatively sharp step and a flat rectangular smooth surface that limits the gas discharge width.
The general feature of both the FIG. 2 regular convex electrode design and the FIG. 3 step electrode design is the uniform laser beam that each provides over the length of the electrodes.
As discussed above, the electrical discharge and electrode configuration of a gas discharge laser strongly defines the size and uniformity of the laser beam. Proper operation of pulsed gas lasers, e.g. excimer (ArF, KrF, XeCl), demands relatively strong pumping intensities (on the order of several MWt/cm3). Only such a strong excitation allows reasonable laser efficiency and pulse-to-pulse laser stability to be reached.
This strong excitation level has disadvantages. The relatively small discharge volume results in a high output laser light energy on the optics/windows of the system, limiting their lifetime. Additionally, the conductivity of the plasmas in the active phase is quite low; obtaining sufficient laser efficiency and good matching between the impedance of the gas discharge plasmas and the electrical circuit demands discharges with the highest ratio of the electrodes' gap and discharge width. However, the best laser beam shape for many applications is rectangular with relatively close beam width and height (if not quadratic). Also, the divergence of the light beam should not differ too much in both directions.
In view of the foregoing, it would be highly desirable to have available a laser discharge configuration that would allow a reduction in the volume of the discharge without significant change in length and in the laser beam cross section.